The present invention relates generally to signal distribution systems and more particularly to apparatus for distributing a time reference clock to all of the individual processors in an active aperture antenna array.
When digital processors are distributed over substantial distances, such that propagation time between elements is significant compared to processing time, then precise synchronization of all processing elements is required to assure their correct interaction. The path over which the synchronizing signals are broadcast must be carefully designed and the waveforms utilized should be selected to obtain the desired accuracy. In addition to operating all elements precisely in synchronism, there are sometimes special circumstances when the time reference must be precisely varied from element to element, for example to compensate for differential delays in signal paths. Such requirements are believed to be common to many applications where distributed processing is a characteristic.
One application which illustrates the magnitude and importance of the problem, is the active aperture antenna array. This array can comprise many thousands of individual radiating/receiving elements spaced over surfaces typically of a few hundred square feet. At each radiating element, a processor controls the phase of the RF signal to steer the antenna beam. The beam can be made extremely agile as the controlling processors are capable of switching in a few nanoseconds. With this agility, time sharing of the antenna to perform varied functions (such as multiple target tracking or communications) and ultra rapid scanning required in the bistatic radar pulse chasing mode, are possible. The beam steering mechanism is typically digitally based, and the transient condition between pointing in one direction and then moving to another direction, introduces disturbances which need to be minimized. These transients are particularly serious during bistatic pulse chasing where scan rates of the order of degrees per microsecond are possible. In this mode, signals are received while scanning by a step/dwell sequence. To minimize the impact of the disturbances created by stepping action, the ratio of times of stepping to dwell should be minimized. This can be accomplished by precise synchronization of the various processor elements.
The time of propagation of a signal in free space is about one nanosecond per foot and with typical antenna apertures of tens of feet, then transmission delays of tens of nanoseconds are possible. A pulse waveform for synchronization of the various processors to one or two nanoseconds will require a transmission path of several hundred megacycle bandwidth. A CW waveform however, occupies negligible bandwidth.